The instant invention relates to a hydraulic pressure-regulating system for directional control of a dual-functioning servo motor, especially for servo-steering of motor vehicles. More specifically, it relates to a system comprising a control valve device which comprises two control pistons serving as valve members for the control of the flow of pressure fluid from one pressure fluid source to the working spaces, or the return flow of pressure fluid from the working spaces of the servo motor to be accomplished in one operation. Each control piston is associated with one working space of the servo motor, and has a means for manually activating the control pistons dependent on one relative movement between a power input member and a power output member. The power output member is connected with the power input member by means of elastic resilient connecting means whereby the power output member is connected with the operational-active portion of the servo motor, for example, with its working piston. The displacement of the working-active portion is based on the control valve device and the control pistons are provided with reaction areas which are stressed by the pressure in the working spaces of the servo motor in such a manner so that each working space is provided with such a reaction area. The activating of the control pistons is made by means of fingers or the like which are connected with the power input member; the fingers engaging or entering the cavities of the control pistons and being in position to carry along the control pistons in both directions of adjustment. A device is provided, which already in the neutral position of the control pistons, produces a predetermined initial pressure in the working spaces of the servo motor.
In a similar pressure control system of the "closed-center construction" (U.S. Pat. No. 3,952,631), an initial pressure is produced in that the control pistons are at the front section activated via a bilateral lever under inter-connection of helical springs, in a manner so that the helical springs are pre-stressed in the neutral position, and the pressure on the reactional areas of the control pistons is counter-effective to the springs. For obtaining a less strong, purely mechanical transmission of the force-component from the power input member to the power output member, push rods are proposed between the control pistons and the lever. These push rods enter into a fixed mechanical contact with the control pistons only after a certain relative movement between lever and control pistons. A utilization of this principle of control pistons which are not activated at the front section but by means of fingers which reach into recesses, or cavities, and with regard to this function are disclosed in the German Pat. No. 1,133,642, would result in a construction in which the srings support themselves on the fingers, and at that side of a control piston which is facing the reaction area would affect the inside of the same against a stop member. The inside would thereby have to be sealed from the reaction area and between fingers and control pistons there would have to exist in both operation directions a certain amount of play. Such a type of construction in and of itself is fully functional and would fulfill also the presented requirements with regard to initial pressure in the neutral position, mechanical power transmission without servo assistance below an operating force limit and automatic return positioning of the steered wheels in the straight-forward position. Nevertheless, there result cases in which the above-described arrangement of the spring has proven to be disadvantageous. One such case developes when the control piston is constructed as a single- or multi-sectional differential piston, and the reaction area represents the front area with the smaller diameter. Since it is often impossible, especially at high initial pressures, to place the spring within such a small diameter, the portion having the larger diameter of the piston would have to be unnecessarily long in shape, thus increasing the structural size of the entire control valve means. The second case connected with disadvantages occurs when a reaction power limitation is proposed with the assistance of a pressure-limiting valve which is arranged within the control piston. Since such a pressure-limiting valve does have certain axial dimensions, additional mounting space for the spring would have to be provided. The same disadvantages result in the utilization of the above-described principles in control valve devices of the open-center-construction. Even though the spring is in such a case not necessary per se for the purpose of producing the initial pressure, because this initial pressure already developes by means of a throttle effect of the flow of pressure medium. The spring, however, keeps the initial pressure constant, that is, independent of variations of the power flow and the viscosity, and thereby improves the steering behavior in various environmental conditions.